There isn't a setting to change how that is drawn - instead, it is managed by the GroupingViewAppearance. The default implementation, GroupingViewDefaultAppearance, contains css that adds that image.

Presently, the html for that image is build in the GroupingView itself, contrary to how we usually apply appearances to factor that out. But since the icon is a background plus some padding, replacing (or removing) that icon via the appearance should be fairly easy.